The present invention relates to centrifugal pumps, and more particularly to centrifugal pumps used for transporting slurries and other abrasive-containing fluids. Specifically the invention concerns centrifugal slurry pumps having physical dimensions such that they are capable of achieving a combination of high efficiency and low wear characteristics not heretofore possible.
A centrifugal pump consists basically of a rotatable impeller enclosed by a collector or shell. As the impeller is rotated, it generates velocity head at the periphery of the shell. The shell collects the velocity head and converts it to a pressure head. There are many configurations within the framework of this basic design. In one common configuration, the flow enters the shell on one side along the axis of rotation of the impeller, that is, the flow enters the shell at a point adjacent to the center of the impeller, referred to as the "eye" of the impeller, while the discharge of the shell is located at a point tangent to the shell outer periphery. The general performance of such a pump is shown in FIG. 1, wherein the flow BEPQ is that at the best efficiency point (BEP), the latter being the highest point of the parabolic efficiency curve. The best efficiency point head (BEPH) is defined as the head at BEP.
The magnitude of the head is largely determined by the impeller diameter, and the flow is mostly affected by the width of the pump and the size of the internal section area. The shell and the impeller tend to work like two nozzles in series, with the impeller generating, and the shell collecting, the head. A change to either will affect the head and the flow. Because both can be varied, more than one combination of variables of impeller and shell dimensions can achieve the same effect.
The magnitude of the peak efficiency is largely determined by the efficiency of the impeller and shell wetted geometry in generating and collecting the head and flow. The location of the BEP is affected in large part by the magnitude (width and depth) of the hydraulic sections. Larger hydraulic sections cause the location of the BEP to move to higher flows.
With regard specifically to slurry pumps, these pumps are subject to high wear due to the abrasive effect of particles in the slurry, which through impact and friction erode the various pump surfaces.
Heretofore, there has been no method of determining or predicting wear except by experience. Empirical data can be useful, except that the observation is global, that is, it does not indicate the individual effect of the different variables of slurry hardness, abrasive size and concentration, resistance of the pump materials of construction, the effect of the pump hydraulic sections and proportions, and the resulting effect on the fluid and slurry particle velocity. Without a means for determining the individual effects of the variables, slurry pump design has heretofore been preoccupied with minimizing wear.
As a consequence, slurry pump hydraulic sections have tended toward sized larger than absolutely necessary in order to keep velocities down, since velocity is a large factor in the wear process. Decreased wear, however, comes at the expense of pump efficiency, since the pump is not operated at or near the BEP. This results in overall increased costs of operation.
Slurry pumps generally have wide impellers to allow passage of large spheres (slurry particles). The thicker metal sections dictated by manufacturing and/or wear considerations require slurry pump impellers to be wider than their equivalent water pump versions. The meridional section (radial section) velocities of a slurry pump impeller are also much lower than an equivalent centrifugal water pump. This means that the hydraulic sections and head losses in the shell play a more significant part in controlling the flow and location of the BEP compared to the more balanced water pumps. Without a tool to analyze and understand wear characteristics, however, it has previously not been possible to optimize the hydraulic energy efficiency and wear performance of slurry pumps.
One of the areas of high wear in slurry pumps is the tongue, which is subject to gouging wear. Tongue wear, or more particularly, wear in the sidewall sections of the tongue, is generally considered to be a three-dimensional phenomenon caused by the higher velocities in the throat and the different velocity in the area between the tongue and the impeller due to recirculating flow.
There is thus a need in the art for slurry pump designs which maximize efficiency without significantly increasing the wear characteristics.